Coherent Micro-mixer

ABSTRACT

A coherent micro-mixer provides a 6-port device having two input ports four output ports. A signal light wave is input into one input port and a reference light wave is input into another input port. The four outputs from the output ports combine to produce interference between the two input light beams, with various relative phase shifts.

This application claims priority Provisional Patent Application Ser. No.61/557,310, titled “Coherent Micro-Mixer” filed Nov. 8, 2011,incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to coherent detection, and morespecifically, it relates to a low cost, compact, andtemperature-insensitive optical hybrid.

2. Description of Related Art

Since the late 1990s, the transport capacities of ultra-long haul andlong-haul fiber-optic communication systems have been significantlyincreased by the introduction of the erbium-doped fibre amplifier(EDFA), dense wavelength division multiplexing (DWDM), dispersioncompensation, and forward error correction (FEC) technologies. Forfiber-optic communication systems utilizing such technologies, theuniversal on/off-keying (OOK) modulation format in conjunction withdirect detection methods have been sufficient to address data rates upto 10 Gb/s per channel.

In order to economically extend the reach and data capacity beyond suchlegacy systems and into next-generation networks, several technologicaladvancements must take place, including but not limited to, 1) adoptionof a differential phase-shift keying (DPSK) modulation format, asopposed to OOK; 2) developments in optical coherent detection; and 3)progress in adaptive electrical equalization technology. In combination,these technologies will boost a signal's robustness and spectralefficiency against noise and transmission impairments.

Such crucial strides in optical signal technology are no longertheoretical possibilities but are feasible solutions in present-dayoptical networking technology. The path for an optical coherent systemhas already been paved by 1) the deployment of phase shift keyingmodulated systems by Tier-1 network providers; and 2) the increasedcomputational capacity and speed of electronic DSP circuits inreceivers, which provides an efficient adaptive electrical equalizationsolution to the costly and difficult optical phase-lock loop. Theseadvances coupled with a commercially feasible optical hybrid solutionwould likely give pause to Tier-1 providers and carriers to reassesstheir earlier rationales for not adopting and implementing an opticalcoherent detection scheme. Perhaps with such advances, optical networkswill begin to realize the benefits already recognized in microwave andRF transmission systems for extending capacity and repeaterlesstransmission distances through coherent detection.

The commercial feasibility of a coherent system for optical signaltransmission was first investigated around 1990 as a means to improve areceiver's sensitivity. In contrast to existing optical direct-detectionsystem technology, an optical coherent detection scheme would detect notonly an optical signal's amplitude but phase and polarization as well.With an optical coherent detection system's increased detectioncapability and spectral efficiency, more data can be transmitted withinthe same optical bandwidth. More over, because coherent detection allowsan optical signal's phase and polarization to be detected and thereforemeasured and processed, transmission impairments which previouslypresented challenges to accurate data reception, can, in theory, bemitigated electronically when an optical signal is converted into theelectronic domain. However, the technology never gained commercialtraction because the implementation and benefits of an optical coherentsystem could not be realized by existing systems and technologies.

Implementing a coherent detection system in optical networks requires 1)a method to stabilize frequency difference between a transmitter andreceiver within close tolerances; 2) the capability to minimize ormitigate frequency chirp or other signal inhibiting noise; and 3) anavailability of an “optical mixer” to properly combine the signal andthe local amplifying light source in local oscillator (LO). Thesetechnologies were not available in the 1990s. A further setback to theadoption and commercialization of an optical coherent system was theintroduction of the EDFA, an alternative low cost solution to thesensitivity issue.

Notwithstanding the myriad challenges, an optical coherent system (alsoreferred to as “Coherent Light Wave”) remains a holy grail of sorts tothe optical community because of its advantages over traditionaldetection technologies. Coherent Light Wave provides an increase ofreceiver sensitivity by 15 to 20 dB compared to incoherent systems,therefore, permitting longer transmission distances (up to an additional100 km near 1.55 μm in fiber). This enhancement is particularlysignificant for space based laser communications where a fiber-basedsolution similar to the EDFA is not available. It is compatible withcomplex modulation formats such as DPSK or DQPSK. Concurrent detectionof a light signal's amplitude, phase and polarization allow moredetailed information to be conveyed and extracted, thereby increasingtolerance to network impairments, such as chromatic disposition, andimproving system performance. Better rejection of interference fromadjacent channels in DWDM systems allows more channels to be packedwithin the transmission band. Linear transformation of a received,optical signal to an electrical signal can then be analyzed using modernDSP technology and it is suitable for secured communications.

There is a growing economic and technical rationale for adoption of acoherent optical system now. Six-port hybrid devices have been used formicrowave and millimeter-wave detection systems since the mid-1990s andare a key component for coherent receivers, in principle, the six-portdevice consists of linear dividers and combiners interconnected in sucha way that four different vectorial additions of a reference signal (LO)and the signal to be detected are obtained. The levels of the fouroutput signals are detected by balanced receivers. By applying suitablebaseband signal processing algorithms, the amplitude and phase of theunknown signal can be determined.

For optical coherent detection, a six-port 90° optical hybrid should mixthe incoming signal with the four quadratural states associated with thereference signal in the complex-field space. The optical hybrid shouldthen deliver the four light signals to two pairs of balanced detectors.Let S(t) and R denote the two inputs to the optical hybrid and

${{S(t)} + {R\; {\exp \lbrack {j( {\frac{\pi}{2}n} )} \rbrack}}},$

with n=0, 1, 2 and 3, represent the four outputs from it. Using the PSKmodulation and phase-diversity homodyne receiver as an illustration, onecan write the following expression for the signal power to be receivedby the four detectors:

${{P_{n}(t)} \propto {P_{S} + P_{R} + {2\sqrt{P_{S}P_{R}}{\cos \lbrack {{\theta_{S}(t)} + {\theta_{C}(t)} - {\frac{\pi}{2}n}} \rbrack}}}},{n = 0},{{\ldots \mspace{14mu} 3};}$

where P_(S) and P_(R) are the signal and reference power, respectively,θ_(S)(t) the signal phase modulation, and θ_(C)(t) the carrier phaserelative to the LO phase. With proper subtractions, the twophotocurrents fed to the TIA's can be expressed as

I _(BD1)∝√{square root over (P _(S) P _(R) )}cos [θ_(S)(t)+θ_(C)(t)];

I _(BD2)∝√{square root over (P _(S) P _(R) )}sin [θ_(S)(t)+θ_(C)(t)];

encompassing the amplitude and phase information of the optical signal.Accordingly, the average electrical signal power is amplified by afactor of 4P_(R)/P_(S). Following this linear transformation the signalsare electronically filtered, amplified, digitized and then processed.Compared to a two-port optical hybrid, the additional two outputs haveeliminated the intensity fluctuation from the reference source (LO).

An optical coherent receiver requires that the polarization state of thesignal and reference beam be the same. This is not a gating item asvarious schemes or equipment are available to decompose and control thepolarization state of the beams before they enter the optical hybrid.Further, certain polarization controllers can be used to provideadditional security functionality for optical coherent systems,preventing third parties from tapping information or data streams byimplementing polarization scrambling and coding techniques.

For laboratory purposes, a 90° optical hybrid has traditionally beenconstructed using two 50/50-beam splitters and two beam combiners, plusone 90° phase shifter. These optical hybrids can be implemented usingall-fiber or planar waveguide technologies; however, both methods havetheir respective drawbacks. Both technologies require sophisticatedtemperature control circuits to sustain precise optical path-lengthdifference in order to maintain an accurate optical phase at theoutputs. In addition, fiber-based devices are inherently bulky and areunstable with respect to mechanical shock and vibration; whereas,waveguide-based products suffer from high insertion loss, highpolarization dependence and manufacturing yield issues. Waveguide-basedproducts are also not flexible for customization and require substantialcapital resources to set up.

Accordingly, a low-cost, temperature insensitive and vibration/shockresistant optical hybrid and method of operating same is desirable andsuch is provided by the present invention.

SUMMARY OF THE INVENTION

An embodiment of the present device is composed of six layers and cantake a variety of forms. Each layer can be a plane parallel fused silicaplate or wafer. The device shown in the figure is a three dimensionaldevice having a substantially square dimension and it third dimension ofthickness that extends into the plane of the drawing sheet. Thisthickness need only be large enough to propagate the beams as discussedbelow. Each layer has a surface that is in proximity to an adjacentlayer, where the surfaces of the two layers that are adjacent form aninterface. For coherent detection, it is important to have a 90-degreephase shift between I1 and Q1. The phase shift can be achieved using alocal heating. In another embodiment, only five layers are provided and

The relative thickness of the central four layers has a ratio of2:1:1:2. These thicknesses need to match the separation distance of thetwo input beams. The skew is an important parameter in coherentdetection. It is the delay between two ports due to the difference inthe optical path.

Various surfaces and interfaces of the coherent micro-mixer are providedwith reflectivity configurations that effectively operate as mirrors oras equivalents to mirrors, or as antireflection (AR) coatings or asequivalents to (AR) coatings, or as beamsplitters or as equivalents tobeamsplitters. The physical dimensions of each of the reflectivityconfigurations are small and need only be at least large enough toperform the intended operation on a beam propagating to thatconfiguration. It can be seen that after propagating through thecoherent micro-mixer, and operated upon by the indicated reflectivityconfigurations the S beam and the L beam are combined into each of thefour exiting beams, wherein the beams are referred to as one of Q2=S−jL,Q1=S+jL, I2=S−L or I1=S+L.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1A shows a function diagram for a 90-degree optical hybrid

FIG. 1B conceptually illustrates the operation of a 90-degree opticalhybrid.

FIG. 2A shows an embodiment of the invention including a 50/50un-polarized beam splitter, a folding prism, a beam shifter, a spacerand a phase shifter.

FIG. 2B shows a rotated view of the embodiment of FIG. 2A.

FIG. 3 shows the layers of the embodiment of FIG. 2A.

FIG. 4 shows reflectivity as a function of air-gap for a fused silicaprism, with light incident at a 45-degree incident angle, where thesolid curve represents P-polarized light and the dashed curve representsS-polarized light.

FIG. 5 illustrates the reflectivity configurations utilized in thepresent invention.

FIG. 6 illustrates an embodiment without the layer 120 of FIG. 2A andincluding a metallic coating an surface 124.

FIG. 7 shows an embodiment of the coherent micro-mixer of the presentinvention.

FIG. 8 shows the two input beams widely separated, and the thickness ofthe central 4 wafers still maintain the ratio of 2:1:1:2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a function diagram for a 90-degree optical hybrid 10. Thisis a 6-port device in that it has two input ports 12, 14 and four outputports 16, 18, 20 and 22. As shown in FIG. 1A, a signal light wave 24 isinput into input port 12 and a reference light wave 26 (from a localoscillator) is input into input port 14. From the four outputs ports 16,18, 20 and 22 exits four outputs 28, 30, 32 and 34 respectively whichare the interferences between the two input light beams, with variousrelative phase shift.

FIG. 1B conceptually illustrates the operation of a 90-degree opticalhybrid. It consists of two 50/50-beam splitters, one each at 40 and 42and two beam combiners, one each at 44 and 46 plus one 90-degree phaseshifter 48. In the practical implementation, this is achieved bywaveguide technology. The size is large and it requires temperaturecontrol to maintain the required optical path length. In addition tothat, typically, the loss is significant and has strong polarizationdependence.

The present invention is an improvement over an earlier optical hybriddesign which was taught in U.S. Pat. No. 7,573,641, titled “Free SpaceOptical Hybrid,” filed Mar. 26, 2007, incorporated herein by reference.FIG. 2A herein shows an exemplary embodiment 100 of the presentinvention and illustrates a method of its operation. In this embodiment,the device is composed of six layers and can take a variety of forms.Each layer can be a plane parallel fused silica plate or wafer. Thedevice shown in the figure is a three dimensional device having asubstantially square dimension and a third dimension of thickness thatextends into the plane of the drawing sheet. This thickness need only belarge enough to propagate the beams as discussed below. Each layer has asurface that is in proximity to an adjacent layer, where the surfaces ofthe two layers that are adjacent form an interface. Specifically, thearticle comprises layers 110, 112, 114, 116, 118 and 120. Layer 110includes a surface 122. Layer 112 includes surfaces 124 and 126. Layer114 includes surfaces 128 and 130. Layer 116 includes surfaces 132 and134. Layer 118 includes surfaces 136 and 138 and layer 120 includessurface 140. A heater is attached to an outer surface 141 of layer 120.For coherent detection, it is important to have a 90-degree phase shiftbetween I1 and Q1. The phase shift can be achieved using a localheating, as shown by the heater 142.

The relative thickness of the layers 112, 114, 116 and 118 has a ratioof 2:1:1:2. These thicknesses need to match the separation distance ofthe two input beams. For instance, if the separation distance between Sand L beams is 250 μm, then the thickness of the layers 112, 114, 116and 118 is 354 μm, 177 μm, 177 μm and 354 μm, respectively. As a result,the four output beams of beam S overlap and interfere with thecorresponding four output beams of beam L. The resulting four outputbeams are I1, I2, Q1, and Q2 (as shown in FIG. 2A) have an equal spacingof 250 um. Therefore, the four output beams can be easily coupled intocommercially available detector arrays, which have a 250 μm pitch.

The skew is an important parameter in the coherent detection. It is thedelay between two ports due to the difference in the optical path. Theskew of I2, Q1, and Q2, relative to I1 is determined by the optical pathlength difference. For a 250 μm pitch, relative to I1, the skew of I2,Q1, and Q2 is 1.2 ps, 2.4 ps and 3.6 ps, respectively. These numbers aredetermined by the design, and can be compensated easily.

Various surfaces and interfaces of the coherent micro-mixer of FIG. 2Aare provided with reflectivity configurations that effectively operateas mirrors or as equivalents to mirrors, or as antireflection (AR)coatings or as equivalents to (AR) coatings, or as beamsplitters or asequivalents to beamsplitters. The particular reflectivity configurationsare discussed, e.g., in FIGS. 2A, 2B and 3 and the operation function ofeach configuration is indicated by the legend of FIG. 2A. Each mirrorcoating or equivalent is indicated by a circle having a single diagonalline. Each AR coating or equivalent is indicated an unfilled circle.Each beamsplitter or equivalent is indicated by a circle having a cross.The configurations and methods for producing them are shown in FIG. 6and discussed infra. The operation of this embodiment is furtherillustrated in FIG. 2A. The physical dimensions of each of thereflectivity configurations are small and need only be at least largeenough to perform the intended operation on a beam propagating to thatconfiguration. It can be seen that after propagating through thecoherent micro-mixer, and operated upon by the indicated reflectivityconfigurations the S beam and the L beam are combined into each of thefour exiting beams, wherein the beams are referred to as one of Q2=S−jL,Q1=S+jL, I2=S−L or I1=S+L. In FIG. 2B, the embodiment of FIG. 2A isrotated in FIG. 2B to horizontally depict the layers. All referencenumbers remain the same as in FIG. 2A.

FIG. 3 illustrates the surface reflectivity configurations of thesurfaces of layers 110, 112, 114, 116 and 118. This figure shows thelayers separated so that the particular reflectivity configuration ofeach layer can be discussed. In the fabrication process, the indicatedreflectivity configurations are provided on their respective surfacesbefore they are affixed together and then the micro-mixer is cut to itsfinal shape and then polished. The surface reflectivity configurationsare indicated according to the legend of FIG. 2A. Each mirror coating orequivalent is indicated by a circle having a single diagonal line andeach is shown with one of reference numbers 151-154. Each AR coating orequivalent is indicated with an unfilled circle and each is shown withone of reference numbers 161-167. Each beamsplitter or equivalent isindicated by a circle having a cross and each is shown with one ofreference numbers 171-174. The vertical alignment of the various surfaceconfigurations should be noted. Note the alignment of the reflectivityconfigurations of FIG. 3 with the reflectivity configurations of FIG.2B. The vertical orientation is provided herein merely for explanationof the alignment of the reflectivity configurations. The ARconfigurations 161, 163 and 168 of layer 116 do not vertically alignwith any other reflectivity configurations. Beamsplitter configurations171 and 172 vertically align. Beamsplitter configuration 173 does notvertically align with any other configuration. Mirror configurations151-153 and AR configuration 162 are aligned. AR configurations 164-166are aligned. Beamsplitter configuration 174 does not align with anyother configuration. The layers shown in FIG. 3 must be bonded together.In one embodiment, the AR coatings are extended over the layer surface,leaving openings for the mirror and beamsplitter reflectivityconfigurations.

FIG. 4 shows reflectivity, for an internal reflection, as a function ofair-gap between a pair of fused silica plates in parallel, with lightincident at a 45-degree incident angle, where the solid curve representsP-polarized light and the dashed curve represents S-polarized light.Notice in the graph that when the air-gap is large (2000 nm), thereflectivity is high (near 0 dB). The lower graph shows that when theair-gap is small (50 nm), the reflectivity is low (near −24 dB forP-polarization).

FIG. 5 illustrates some of the reflectivity configurations utilized inthe present invention. The air gap between the two glass fused silica)elements 200 and 202 is about 2 μm in this example. There are threeexemplary regions provided in the figure. The region on the left side ofthe figure comprises a beamsplitter coating 210. Methods for fabricatingsuch coatings are known in the art. Notice that the coating touches thetwo pieces of glass 200 and 202. The beamsplitter can be used to connecttwo layers together. This area can be used for beam splitting or beamcombining. Beam 204 is split by the coating to produce a transmittedbeam 204 a and a reflected beam 204 b. In the center region, a SiO2coating 214 extends from glass element 200 to almost touch the top glasselement 202. If the air-gap is less than 40 nm, more than 99% of thelight from bean 206 will pass through the combined glass elements toproduce a transmitted beam 206 b. In such case, this area effectivelyoperates as an AR coated surface. In the present invention, it is oftenbeneficial if the AR coating touches both surfaces, and can thus be usedas a bonding mechanism between two layers. Both the beamsplitter coatingand the AR coating can be extended to cover enough area between the twolayers in order to provide a stable connection. Care must be given, inthe case of the beamsplitter, that its surface area does notinadvertently intersect a beam line. As illustrated with beam 208, wherethe air-gap is about 2 μm or greater, more than 97% of light will bereflected to form beam 208 b. It can be considered as mirror. Thoseskilled in the art will recognize that the beamsplitter coating and theAR coating can be lithographically produced. This figure shows aninterface between two wafers having two BS coatings, and one AR coating.Obviously, the AR coating can be coated on one wafer, and the BS coatingon the other. Both AR and BS coatings have about the same thickness.After coating, the two wafers are bonded with optical contact means. Itshould be noted that even if the AR thickness is slightly less than thatof BS coating, the reflectivity at AR coating is still very small (e.g.,−30 dB), as explained above. The open space that has no coating canserve as HR or TIR coating.

FIG. 6 illustrates an embodiment that represents a modification of theembodiment of FIG. 2A. Like elements are labeled with identicalreference numbers. This embodiment differs from the embodiment of FIG.2A, except that (i) it lacks layer 120, (ii) it lacks a portion of layer110 and (iii) it replaces the mirror coating at the interface betweensurfaces 122 and 124 with a mirror coating 160 (e.g., a metalliccoating) on surface 124.

As it is well known, the phase change on reflection by total internalreflection (TIR) is sensitive to the angle of incidence (AOI). If eachof the two interference beams experience TIR in its path, due to thesymmetry in the design, the angular dependence of the phase change onreflection is cancelled. On the other hand, if only one of the two beamshas one more TIR in its path than the other beam has, the phasedifference between the two beams will change with the AOI. Again, thisembodiment lacks the heater of FIG. 2A and replaces the TIR coating onsurface 124 with a metallic coating. Therefore, by tilting the device,the phase difference can be adjusted between I1 and Q1 to satisfy the90-degree phase shift requirement. Because I1 and I2 are 180 degree outof phase (which is true also for Q1 and Q2), the phase differencebetween I2 and Q2 is automatically equal to 90 degree approximately,when the phase difference between I1 and Q1 is adjusted to satisfy the90-degree requirement.

FIG. 7 shows a demodulator having two coherent micro-mixers 100 and100′. Micro-mixer 100 is the embodiment of FIG. 2A and micro-mixer 100′is a duplicate of micro-mixer 100. The demodulator further includes apolarizing beamsplitter 300 and a wave plate 310 for the demodulation ofDP-QPSK signals.

It should be noted that the phase change on reflection for TIR inside anuncoated glass substrate is dependent not only on AOI, but also on thepolarization. Similarly, the phase change by a BS coating is alsodependent on the polarization. Therefore, in FIGS. 2A, 2B and 6, the twoinput beams should have the same polarization. Preferably, thepolarization at the input port is either P-polarization (parallel to theplane of the page) or S-polarization (perpendicular to the plane of thepage).

In the Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK)demodulation, the signal beam has dual polarizations. The localoscillator beam is orientated accordingly. Therefore, a polarizationbeam splitter (PBS) is used to separate the polarizations, and awaveplate is used to rotate one of the polarization, as shown in FIG. 7.The polarization of the transmitted beams after the PBS is rotated bythe waveplate 310.

It should be also noted that a glass substrate can be coated such thatthe phase change on reflection for the TIR is only dependent on AOI, butnot on the polarization. Similarly, a BS can be made to benon-polarization dependent See U.S. Pat. No. 7,145,727 incorporatedherein by reference. Using the special TIR coating and BS coating, thewaveplate is not needed.

Finally, due to the highly symmetric structure of the current invention,the device is athermal. The skew among the four exit beams is notsensitive to the environment temperature. The heater in FIG. 2A is usedas a phase shifter if the skew between I1 and Q2 needs to be adjusted.

The invention is not limited to the disclosed configurations. Variousconfigurations can be made to have the functionality of the currentinvention. For example, FIG. 8 shows an embodiment where the two inputbeams are widely separated, and the thickness of the central 4 wafersstill maintain the ratio of 2:1:1:2. In this case, the input beam L isfirst reflected by either a TIR or metallic coating.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

We claim:
 1. A coherent micro-mixer, comprising: a first input port forreceiving a first beam of light (S beam) into said micro-mixer; a secondinput port for receiving a second beam of light (L beam) into saidmicro-mixer; a first interface, a second interface, a third interface, afourth interface and a fifth interface, wherein said S beam canpartially transmit through said first interface and said secondinterface and then exit said micro-mixer at a first output port, whereinsaid S beam can partially reflect at said second interface and then bereflected at said first interface to then exit said micro-mixer at asecond output port, wherein said S beam can partially reflect from saidfirst interface and then be reflected by said fifth interface and thenpartially transmit through said second interface and then exit saidmicro-mixer at a third output port, wherein said S beam can be partiallyreflected from said first interface, and then be partially reflected bysaid second interface and then exit said micro-mixer at said fourthoutput port, wherein said L beam can partially reflect from said thirdinterface and then partially reflect from said second interface and theexit said micro-mixer at said first output port, wherein said S beam andsaid L beam are combined to produce a combined beam I1, wherein said Lbeam can be reflected at said first interface and then exit saidmicro-mixer at said second output port, wherein said S beam and said Lbeam are combined to produce a combined beam I2, wherein said L beam canbe reflected at said fourth interface and then be partially reflected atsaid second interface and then exit said micro-mixer at said thirdoutput port, wherein said S beam and said L beam are combined to producea combined beam Q1, wherein said L beam can be reflected at said firstinterface and then exit said micro-mixer at said fourth output port,wherein said S beam and said L beam are combined to produce a combinedbeam Q2, a phase shifter for producing a phase difference between I1 andQ1 of about 90 degrees and a phase difference between I1 and Q2 of about270 degrees.
 2. The coherent micro-mixer of claim 1, wherein said phaseshifter comprises a heater.
 3. The coherent micro-mixer of claim 1,wherein said phase shifter comprises a metallic reflector operativelypositioned at said fourth interface to reflect said L beam.
 4. Acoherent micro-mixer, comprising: a first input port for receiving afirst beam of light (S beam) into said micro-mixer; a second input portfor receiving a second beam of light (L beam) into said micro-mixer;means for combining said S beam and said L beam into four combined beamsI1, I2, Q1 and Q2, wherein I1 and I2 have a phase difference of about180 degrees; and a phase shifter for producing a phase differencebetween I1 and Q1 of about 90 degrees and a phase difference between I1and Q2 of about 270 degrees.
 5. The coherent micro-mixer of claim 4,wherein said phase shifter comprises a heater.
 6. The coherentmicro-mixer of claim 4, wherein said phase shifter comprises a metallicreflector operatively positioned at said fourth interface to reflectsaid L beam.
 7. A method for fabricating a coherent micro-mixer,comprising: providing a first input port for receiving a first beam oflight (S beam) into said micro-mixer; providing a second input port forreceiving a second beam of light (L beam) into said micro-mixer;providing a first interface, a second interface, a third interface, afourth interface and a fifth interface, wherein said S beam canpartially transmit through said first interface and said secondinterface and then exit said micro-mixer at a first output port, whereinsaid S beam can partially reflect at said second interface and then bereflected at said first interface to then exit said micro-mixer at asecond output port, wherein said S beam can partially reflect from saidfirst interface and then be reflected by said fifth interface and thenpartially transmit through said second interface and then exit saidmicro-mixer at a third output port, wherein said S beam can be partiallyreflected from said first interface, and then be partially reflected bysaid second interface and then exit said micro-mixer at said fourthoutput port, wherein said L beam can partially reflect from said thirdinterface and then partially reflect from said second interface and theexit said micro-mixer at said first output port, wherein said S beam andsaid L beam are combined to produce a combined beam I1, wherein said Lbeam can be reflected at said first interface and then exit saidmicro-mixer at said second output port, wherein said S beam and said Lbeam are combined to produce a combined beam I2, wherein said L beam canbe reflected at said fourth interface and then be partially reflected atsaid second interface and then exit said micro-mixer at said thirdoutput port, wherein said S beam and said L beam are combined to producea combined beam Q1, wherein said L beam can be reflected at said firstinterface and then exit said micro-mixer at said fourth output port,wherein said S beam and said L beam are combined to produce a combinedbeam Q2, providing a phase shifter for producing a phase differencebetween I1 and Q1 of about 90 degrees and a phase difference between I1and Q2 of about 270 degrees.
 8. The method of claim 7, wherein saidphase shifter comprises a heater.
 9. The method of claim 7, wherein saidphase shifter comprises a metallic reflector operatively positioned atsaid fourth interface to reflect said L beam.
 10. A method forfabricating a coherent micro-mixer, comprising: providing a first inputport for receiving a first beam of light (S beam) into said micro-mixer;providing a second input port for receiving a second beam of light (Lbeam) into said micro-mixer; providing means for combining said S beamand said L beam into four combined beams I1, I2, Q1 and Q2, wherein I1and I2 have a phase difference of about 180 degrees; and providing aphase shifter for producing a phase difference between I1 and Q1 ofabout 90 degrees and a phase difference between I1 and Q2 of about 270degrees.
 11. The method of claim 10, wherein said phase shiftercomprises a heater.
 12. The method of claim 10, wherein said phaseshifter comprises a metallic reflector operatively positioned at saidfourth interface to reflect said L beam.